Spain uses C.I.P proof regs as does most all of Europe.
They run 2 proof rounds through each revolver.
I'm fairly certain the handgun proof rounds currently have a 30% increase in chamber pressure over what is allowed in commercial ammo under CIP regs.

Proof round(s) caliber must match the caliber the manufacturer has stamped onto the firearm.
So even if the manufacturer used 357 chambered cylinders (interchangable/usable from another model?) in this case, the revolvers would have received a 38Special proofing at Eibar Proof House because of the manufacturers marked caliber designation.

Not knowing what if any difference there may be between the model 960 in 38special and a similar one (if there was one made by Astra)chambered in 357Mag as far as factory heat treat or steel alloy is concerned for the frame & barrel,,I'd use the 960 as a 38special.
Just my opinion..

A 357 can generate twice the breech pressure + as a 38 special no matter whos proof & service pressure system is referenced.